New method for construction kinetic diagram by fatigue crack growth rate tests

Аuthors

Krupenin A. M.

A Lyulka Experimental Design Bureau – Branch of PJSC “UEC-UMPO”, 13, Kasatkina str., Moscow, 129301, Russia

e-mail: Zeus-RUSS@yandex.ru

Abstract

The article proposes a new method for kinetic diagram plotting of the fatigue crack growth rate by experimental data.

The article is divided into nine parts.

The first part discusses the work relevance, as well as adduces links to the previously conducted research on this issue.

The second part provides general description of processing results of the experiments on fatigue crack growth rate.

The third part discusses specifics and limitations of the current method of results processing of the experiments on fatigue crack growth rate.

The fourth part gives and substantiates assumptions on the qualitative characteristics of experimental data.

The fifth part describes the proposed method for the results processing of the experiments on fatigue crack growth rate. Special basic functions with which processing is performed, are being introduced.

The sixth part performs the studies of the derivatives and integrals of basic functions.

The seventh part describes the methods for determining characteristics of the kinetic diagram. These are the boundaries of the second section, coefficients of the Paris equation and coefficients of the equation approximating the first section of the kinetic diagram.

The eighth part demonstrates the results of the kinetic diagram characteristics computing, as well as provides an example of the operating time computing for a compact sample.

The ninth part presents inferences on the work being accomplished.

1. The causes limiting accuracy of the classical methods for plotting a kinetic diagram are shown. The article demonstrates that the relative error in determining the FCGR through the ratio of finite differences is rather significant and it is not constant in the value of the argument of the V(N) function. Thus, computing the FCGR through the finite differences may lead to a significant error in defining the kinetic diagram and its characteristics.

2. The author introduces and substantiates the assumptions on the qualitative mathematical properties of experimental curves and kinetic diagram. The following properties such as monotony, convexity and the presence of an asymptote are being supposed for the experimental dependence l(N). For dependencies V(N) and V(ΔK(l)), we assume the following properties: smoothness, monotony, and the presence of an asymptote (for V(N)).

3. A new method for plotting a kinetic diagram of the FCGR from the SIF range based on the results of an experiment at the FCGR, with account for the qualitative characteristics of the experimental data has been developed. The said method allows obtaining a kinetic diagram not in the form of a scatter of points, but in the form of a smooth function, which, in its turn, allows considering the kinetic diagram characteristics, as well as some specifics of its behavior, including the first section of the diagram, in more detail.

4. Basic functions, by which experimental data is processed on the FCGR in the proposed method are introduced and studied.

5. Methods for obtaining the kinetic diagram characteristics, describing both the stage of steady growth and the first section of the kinetic diagram, have been developed.

6. The article demonstrates that the first stage of the crack growth may occupy a large part of the cyclic durability.

7. It was demonstrated that a less conservative solution than the solution obtained using only the Paris equation is being obtained with account for the first section of the kinetic diagram when computing the operating time.

Keywords:

kinetic diagram, fatigue crack growth rate, Paris law, kinetic diagram first part

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